A control system utilizes a device or set of devices to regulate, direct or manage the behavior of systems or other devices. There are various types of control systems, including closed loop systems and open loop systems. Closed loop systems employ feedback, whereas open loop systems use prearranged schemes instead of feedback.
Control systems are used in a multitude of applications in electrical and electronic systems and devices. For example, control systems are used in wireless transmitters and receivers, speaker systems, televisions, computers, and many other applications. One function of control systems in these applications is to ensure that components, such as linear or nonlinear devices, achieve a desired output. For example, a control system may be responsible for controlling the generation of an output signal with a specific power level and waveform quality corresponding to a specific input.
The design of control systems must account for specific requirements and standards that regulate the production, sale, and operation of electrical devices and equipment. For example, wireless standards such as Global System for Mobile communications (GSM), Enhanced Data Rates for GSM Evolution (EDGE), code division multiple access (CDMA), Universal Mobile Telecommunications System (UMTS), and many others include strict spectral requirements. As a result, electrical components must preserve waveform quality at various power levels to meet these strict requirements.
In nonlinear devices, such as power amplifiers as well as other devices, waveform quality typically increases with a more linear output response. However, some nonlinear devices operate more efficiently when the output response is nonlinear—for example, when a power amplifier approaches saturation. As a result, there is often a tradeoff between waveform quality and efficiency. For example, if a nonlinear device is set to operate in linear regions to meet quality standards or requirements, then power consumption and current drain may be degraded because the device is operating at a lower efficiency level. Alternatively, if a nonlinear device approaches saturation or starts to exhibit nonlinear qualities (which may improve efficiency), the waveform quality may be degraded and may not meet the specific requirements and standards.
To improve waveform quality from a nonlinear device operated near saturation, the input to the device may be pre-distorted to account for the nonlinearity of the device. For example, the device may be tested at the time of manufacturing to determine the its nonlinear response. Based on this measurement, an inverse pre-distortion function may be programmed into a lookup table associated with the device. During subsequent operation of the device, the lookup table may be used to pre-distort the device input to achieve a desired linear output. A potential disadvantage of this open-loop control is that the nonlinearities of the device may change over time. As a result, the pre-distortion function may no longer match the actual nonlinearities of the device, and the device may fail to provide the desired linear output.
Closed-loop control also may be used to pre-distort the device input to achieve a desired linear output. For example, a closed-loop approach may include a feedback loop to provide real-time information about the device output. Based on a comparison of actual device output and the desired output, the input signal may be adjusted by calculating a real-time pre-distortion function for the device input to achieve the desired linear output. This type of closed-loop control may continue to provide a desired linear output even if the nonlinearities of the device change over time. A potential disadvantage of this approach, however, is that it may increase power consumption due to the increased logic or circuitry required for the feedback loop and real-time calculation of the pre-distortion function.
Accordingly, there is a need for control systems and methods that provide reliable and efficient input pre-distortion for nonlinear devices. There is a further need for control systems and methods that provide for such reliable and efficient input pre-distortion even when the nonlinearities of a device change over time.